Enhanced 2.0 μm emission and lowered up-conversion emission in Ce3+/Yb3+/Ho3+ tri-doped Na5Y9F32 single crystals

Abstract

In this paper, the Na5Y9F32 single crystals tri-doped with ~3 mol% Yb3+/~1 mol% Ho3+, and different concentrations of Ce3+ (0, 0.2, 0.25, 0.35 and 0.4 mol%) were successfully grown by the modified Bridgman method. The absorption spectra, fluorescence spectra and decay curves of co-doped Yb3+/Ho3+ Na5Y9F32 single crystals with different concentrations of Ce3+ ions were measured. The intensity parameters of Ho3+ ions were calculated according to Judd–Ofelt theory and measured absorption spectra. The emission spectra showed the up-conversion (UC) emission intensity at 545, 657, and 755 nm reduced significantly as the addition of Ce3+. When the concentration of Ce3+ reached about 0.25 mol%, the maximum fluorescence intensity of ~2.0 μm was obtained, and the emission cross section reached 2.29 × 10−20 cm2. The mechanism of up-conversion reduction and ~2.0 μm emission enhancement in Ce3+/Yb3+/Ho3+ tri-doped Na5Y9F32 crystals was discussed from their energy level diagrams and optical spectra. The cross-relaxation (CR) energy transfer efficiency between Ho3+ and Ce3+ ions were 36.3% calculated from the measured lifetime of Yb3+ /Ho3+ co-doped and Ce3+/Yb3+/Ho3+ tri-doped samples. Analysis on the fluorescence dynamics indicated that electric dipole-dipole is dominant for the energy transfer from Ho3+ to Ce3+ ions. The results show that Ce3+/Yb3+/Ho3+ tri-doped Na5Y9F32 crystals has certain application prospects in improving the performance of Ho3+ ~2.0 μm solid-state laser.